1. Field of the Invention
The present invention relates to a thixocasting process, and particularly, to an improvement in a thixocasting process which involves subjecting an aluminum alloy material to a heating treatment to prepare a semi-molten aluminum alloy material having a solid phase (which is a substantially solid phase and this term will also be applied hereinafter) and a liquid phase coexisting therein, pouring the semi-molten aluminum alloy material into a cavity in a casting mold under pressure, and then solidifying the semi-molten aluminum alloy material under pressure.
2. Description of the Related Art
There is a conventionally known thixocasting aluminum alloy material which has a hypo eutectic crystal composition and a characteristic that a first angled endothermic section appearing due to the melting of a eutectic crystal and a second angled endothermic section appearing due to the melting of a component having a melting point higher than a eutectic point in a differential calorimetric curve. In a thixocasting process using such material, if the temperature of a drop-end point in the first angled endothermic section is represented by T.sub.2, and the temperature of a peak in the second angled endothermic section is represented by T.sub.3, the casting temperature of the semi-molten aluminum alloy material is set in a range of T.sub.2 .ltoreq.T.ltoreq.T.sub.3.
The reason why the casting temperature T is set at a relative high level in this manner is for the purpose of decreasing the solid phase proportion in the semi-molten aluminum alloy material to improve the castability of the latter.
In this case, the metallographic structure of the resulting aluminum alloy cast product comprises an .alpha. phase formed by the solidification of the solid phase, and a matrix, i.e., a .alpha.-.beta. eutectic crystal phase formed by the solidification of the liquid phase. The aluminum alloy cast product has a mechanical characteristic depending upon the metallographic structure thereof.
To further enhance the mechanical characteristic of the aluminum alloy cast product, it is contemplated to cause a .beta. phase to be finely precipitated in the matrix. However, it is impossible to realize the fine precipitation of the .beta. phase by the conventional process.
To achieve a further increase in strength and a reduction in weight of the aluminum alloy cast product, an Al-Mg.sub.2 Si based alloy material as an aluminum alloy malleable material may be used as a thixocasting aluminum alloy material. In this case, if the content of Mg in the Al-Mg.sub.2 Si based alloy material, i.e., the Mg.sub.2 Si content is too small, the liquid phase amount is insufficient due to the appearing of only a single angled endothermic section in the differential calorimetric curve. For this reason, a shrink cavity is liable to be produced around a spherical .alpha.-Al portion of the aluminum alloy cast product. On the other hand, if the Mg.sub.2 Si content is too large, the following problem is encountered: a large amount of bulky brittle Mg.sub.2 Si crystals exist in the aluminum alloy cast product, and the liquid phase content is increased, so that the amount of hydrogen dissolved in the liquid phase is increased. For this reason, blow holes are liable to be produced in the aluminum alloy cast product, thereby causing a reduction in strength of the aluminum alloy cast product.
Further, a thixocasting aluminum alloy material is also used which contains a relatively large amount of Sr added thereto. The reason why the Sr content is defined is to reliably finely divide the metallographic structure of the matrix produced by the solidification of the liquid phase and to increase the electric resistance value of the aluminum alloy material to enhance the red-heated degree of the semi-molten aluminum alloy material by an induction heating. However, there is a problem that if the amount of Sr added and the shear rate of the semi-molten aluminum alloy material in the cavity are unsuitable, the toughness of the aluminum alloy cast product is largely lost.
Yet further, there is a conventionally known thixocasting aluminum alloy material having a characteristic that a first endothermic section appearing due to the melting of a eutectic component and a second endothermic section appearing due to the melting of a component having a melting point higher than a eutectic point exist in a differential calorimetric curve. If the maximum value of the distance between a straight line interconnecting a melt-start point of the eutectic component and a melt-end point of the high-melting component and the first endothermic section is represented by t, and the maximum value of the distance between such straight line and the second endothermic section is represented by u, the ratio u/t of the maximum value u to the maximum value t is in a range of u/t&gt;0.1.
However, the known semi-molten aluminum alloy material suffers from the following problem: The high-melting component is colloidal and is in a softened state. In addition, since the ratio u/t is in the range of u/t&gt;0.1, the amount of heat of the solidification of the high-melting component is large and hence, the time until the solidification of such component is relatively long. Due to these facts, the high-melting component is agglomerated to cause a deterioration in the fluidity of the semi-molten aluminum alloy material and hence, casting defects such as a cold shut are liable to be produced in an aluminum alloy product.